Exterior Paint Calculator
Coverage for siding, stucco and brick with big openings deducted.
Interior painting
How much paint do I need for a room?
Enter the room length, width and ceiling height and the paint calculator returns the exact gallons or liters needed per coat — doors and windows are deducted, and a "wall + ceiling" option is built in. One gallon of interior paint covers about 350 square feet on smooth drywall, so a typical 12 × 12 ft bedroom needs roughly 2 gallons for two coats.
Whether you are repainting one bedroom or a whole house, this estimator runs the same math a professional painter uses to quote a job: wall and ceiling area minus doors and windows, multiplied by the number of coats and divided by the coverage per gallon (350 sq ft by default). You get the total in gallons and liters, plus a can-count buying suggestion, before you head to the paint store.
Frequently asked questions
How many gallons of paint do I need for a 12×12 room?
About 2 gallons for two coats on the walls, or 2–3 gallons if you also paint the ceiling. Enter 12 × 12 into this calculator and it returns the exact number for your door and window setup.
How many square feet does one gallon of paint cover?
One gallon of interior latex paint covers roughly 350–400 sq ft per coat on smooth, primed drywall. Textured walls, unpainted drywall and heavier colors reduce that to about 250–300 sq ft per gallon.
Should I subtract doors and windows when measuring walls?
Yes — a standard interior door is about 21 sq ft and a window about 15 sq ft. Deducting them keeps you from overbuying; the calculator does this automatically.
Is primer counted as a paint coat?
No. The calculator counts finish coats. If you are painting bare drywall or covering stains, buy one extra gallon of primer and treat it like an additional coat when estimating total volume.
How do I get an even finish when I buy paint in two cans?
Pour both gallons into a single 5-gallon bucket and stir well before starting ("boxing" the paint). This makes the two cans indistinguishable, which avoids visible color differences across walls.
How many liters of paint do I need for a room?
One US gallon equals 3.785 liters, so the 2 gallons a typical 12 × 12 ft bedroom needs for two coats is about 7.6 liters. The result panel always lists the metric equivalent in liters and square meters alongside the gallons.
Why does painting a dark color over a light wall need extra paint?
A dark or bold color usually needs a tinted primer plus two full coats for even coverage, and per-gallon coverage can drop to roughly 250–300 sq ft. Budget an extra gallon over the standard estimate for a deep color change.
Paint Calculator
Imperial (ft / gal)
ft
ft
ft
Sq ft covered per gallon per coat.
× 21 sq ft
× 15 sq ft
Paint the ceiling too?
Coats of paint
Enter your room dimensions to see the result.
–
gallons
Rates are typical published coverage values — always check the label on your paint can.
How it works
How to use this paint calculator
1Measure the room
Measure length, width and ceiling height in feet. For a standard room this is enough — the tool calculates the whole wall area from the perimeter.
- 8 ft ceilings are the US default; adjust for vaulted or tray ceilings.
- For L-shaped rooms, split them into two rectangles and add the areas.
- Ceiling paint uses roughly one extra gallon per two coats of wall paint.
2Set doors, windows and coats
Door and window openings don't need paint — the tool deducts 21 sq ft per door and 15 sq ft per window. Then choose how many coats you will roll.
- Two coats: recommended for an even, rich color.
- Three coats: covering a much darker color or new bare drywall.
- Rough, textured surfaces drink more paint — choose "Textured".
3Read the result and buy
The main number is the exact volume of paint the walls need at the coverage you picked. The note below tells you how many full 1-gallon cans to buy — you can always mix paint from two cans in one bucket to guarantee an even finish.
The math
Paint quantity formula
Wall area = 2 × (length + width) × wall height − (doors × 21 sq ft) − (windows × 15 sq ft) (+ ceiling = length × width, when selected)
Gallons needed = (wall area × coats) ÷ coverage per gallon
Worked example — a 12 × 10 ft room with 8 ft walls, 1 door, 2 windows, ceiling included, two coats at 350 sq ft/gal:
Walls: 2 × 22 × 8 = 352 − 21 − 30 = 301 sq ft; Ceiling: 120 sq ft; Total 421 sq ft × 2 coats = 842 ÷ 350 = 2.4 gal → buy 3 gallons.
Walls: 2 × 22 × 8 = 352 − 21 − 30 = 301 sq ft; Ceiling: 120 sq ft; Total 421 sq ft × 2 coats = 842 ÷ 350 = 2.4 gal → buy 3 gallons.
Coverage guide
How far does a gallon of paint go?
Estimates are for two coats of interior paint on typical surfaces. Door and window openings deducted.
| Room size | Walls only | Walls + ceiling |
|---|---|---|
| 10 × 10 ft | 1 gal | 2 gal |
| 12 × 12 ft | 2 gal | 2–3 gal |
| 12 × 16 ft | 2–3 gal | 3 gal |
| 16 × 20 ft | 3 gal | 4 gal |
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